This invention relates broadly to UF.sub.6 -recovery process entailing direct conversion of gaseous UF.sub.6 to a solid in a desublimer (cold trap), the desublimer subsequently being heated to convert the UF.sub.6 to a fluid. More particularly, it relates to a novel process for recovering process gas from a UF.sub.6 gas-centrifuge cascade. The invention is a result of a contract with the United States Department of Energy.
The invention was made in the course of experiments conducted to determine the most suitable method for recovering UF.sub.6 from a projected gas-centrifuge cascade of the kind described briefly in a report entitled "United States Gas Centrifuge Program for Uranium Enrichment", Union Carbide Corporation, Nuclear Division, Oak Ridge, Tenn. (June 2, 1981). Referring to FIG. 1, the cascade 9 is designed to receive a feed stream 11 of process gas (UF.sub.6) and to separate the same into a .sup.235 U-depleted waste stream 13 and a .sup.235 U-enriched product stream 15. As shown, the cascade comprises a series of stages, each including a plurality of gas centrifuges 16. Each centrifuge is provided with an inlet for feed gas and outlets for enriched gas and depleted gas, respectively. As indicated, the centrifuges for a given stage are connected in parallel.
The product stream 15 from the cascade is virtually pure gaseous UF.sub.6 which is at a pressure below atmospheric. The amount of noncondensable gases (e.g., nitrogen) in the stream normally is below 0.2% by volume. Because UF.sub.6 is desublimable (it condenses directly from a gas to a solid at temperatures below 55.6.degree. C. under 760 mm pressure), tests were conducted to establish whether the cascade product could be recovered efficiently by (a) directing the product stream through a desublimer to condense the UF.sub.6 as a solid; (b) heating the UF.sub.6 -loaded desublimer to liquefy the solid UF.sub.6 ; and (c) recovering the liquid UF.sub.6.
The tests were conducted by directing a stream of virtually pure, gaseous UF.sub.6 at subatmospheric pressure through basically conventional cold traps having means for external cooling and heating. The typical trap comprised a horizontally oriented metal tube having a uniform diameter and containing internal, longitudinally extending cooling fins. Strain gages were mounted to the exterior wall of the trap at various points. Typically, desublimation of the UF.sub.6 was effected at -100.degree. F., at which temperature the UF.sub.6 deposited on the inside wall of the tube as an annular, crystalline layer. The layer was up to three inches thick and extended extending for most of the length of the tube. Subsequently, this layer was melted by heating the trap to 175.degree. F., well above the triple point for UF.sub.6. In the course of these tests, it was discovered that excessive mechanical stresses often were generated in the traps during the heating cycle. In a test conducted with an aluminum-alloy trap, the stresses were sufficiently large to split a longitudinal welded seam of the trap.
In an attempt to avoid the development of such large stresses, a UF.sub.6 cold trap was designed with internal wedge-shaped fins to act as stress breakers. Tests conducted with UF.sub.6 showed that the fins did not provide sufficient stress relief.
The use of cold traps (desublimers) to remove UF.sub.6 from gas streams consisting of UF.sub.6 and about 30%-76% nitrogen is disclosed in the following co-assigned patents: U.S. Pat. No. 3,853,507 (Dec. 10, 1974); U.S. Pat. No. 3,859,507 (Jan. 14, 1975); and U.S. Pat. No. 3,958,808 (Jan. 14, 1975). The UF.sub.6 is trapped as a solid, following which the traps are heated to permit recovery of the UF.sub.6 as a liquid.